If the degree of compaction achieved with a vibrating compacting tool can be measured simply and continuously, and if the frequency and amplitude of the vibration of the compaction tool, as well as the speed with which the tool is moved across the foundation, can be varied, it would be possible to control the compaction tool with the aim of attaining optimal compaction. The danger of terminating compaction before a sufficient degree of compaction has been attained, or continuing compaction although a sufficient degree of compaction has already been attained, could be minimized. There has therefore long existed a need for a simple, inexpensive and reliable continuously measuring compaction degree meter for vibrating tools. In the patent literature there are many more or less different proposed designs of compaction degree meters. Among those that may be of interest as a background to the present invention, the ones described in British Pat. No. 1,372,567 and U.S. Pat. Nos. 3,599,543 and 4,103,554, for example, may be mentioned.